Method and Apparatus for Feeding Chemicals Into a Process Liquid Flow

ABSTRACT

A method and apparatus for feeding a retention chemical together with an additive, which may be another chemical or for example a mineral, to paper suspension flow in a paper machine. The method and the apparatus are applicable to the simultaneous feeding of an additive of the paper manufacture, such as a filler, together with a retention chemical to paper pulp.

The present invention relates to a method and an apparatus for feeding chemicals into a process liquid flow. An application of a preferred embodiment of the method and the apparatus of the present invention is feeding retention chemical/chemicals together with an additive, which may be another chemical or for example a mineral, to paper pulp suspension flow to be fed to a paper machine. The method and the apparatus of the invention are particularly well applicable in feeding an additive of the paper manufacture, such as filler, together with a retention chemical, to paper pulp essentially simultaneously.

At first, chemicals used in the paper manufacture and their properties will be discussed.

Retention chemicals are chemical agents the purpose of which is to bind various substances carried by the paper pulp suspension either to each other or especially to the fibers of the paper pulp so that the substances in question would remain in the product to be manufactured, the so-called web and would not be flushed away from it when the paper web is dewatered at the wire section of the paper machine. An operating principle of the retention chemicals is based on the electric charge typical of the particles in paper pulp.

For example it can be thought that the typical charge of the paper pulp fibers is negative and that of the additive or filler used in the paper manufacture is also negative. If efforts are made to cause these additives/fillers to remain in the paper to be produced, the success is poor as the fibers having the same electric charge reject these additives/fillers. Then the fibers in a way force the substances in question to the water phase from which they with very high probability end up in the white water filtered out in the wire section of the paper machine.

The situation can be corrected by feeding to the paper pulp retention chemical which has a positive specific charge and which thus adheres both to the fibers and to the additive/filler in question thus binding them to each other. Performed tests have further shown that the longer the retention chemical is in contact with for example the fibers the weaker its retention ability becomes. This is believed to be due to the feature that the electric charge of the fiber attracts all the time the retention chemical molecule so that with time practically the whole molecule rests against the fiber whereby the internal electric charge of the retention chemical is in a way discharged to the fiber and the substance is left without a charge or in the worst case adopts the charge of the fiber. Naturally the additive/filler does not then even try anymore to get in touch with the retention chemical but stays free in the paper pulp suspension.

In the literature the retention chemicals are in most cases understood to be cationic or anionic acryl amid copolymers. These have been found to improve efficiently the retention of fines in the paper formation. However, there are many alternative substances and additives which may also be used to improve the retention. As an example may be mentioned, among others, combinations of two different polymers or copolymers sometimes having even different electric charges, used in sequence, or for example the use of a high-mass cationic polymer in combination with an anionic micro particle such as colloidal silica, bentonite or micro polymer, introduced later, or the various options provided by polyethylene oxide. The purpose of the retention chemical is, in addition to retaining for example the fillers in the web, also to maintain adequate tidiness of the paper machine, to provide uniform quality in the Z direction of the web, and to ensure infiltration ability.

The retention chemicals play a central role in the paper manufacture and the quality of the end product. For example an excessive dose of the retention chemical results in flocks in the end product which are seen as uneven quality of the product. Thus, the aim is to dose only the necessary amount of the retention chemical in order to achieve the goals described above, and not more. However, some chemicals such as ASA (explained later) used as an adhesive require a relatively high retention chemical dose whereby a homogenous mixing of the chemical to the paper pulp is naturally of primary importance. ASA, which is not retained in the fibers, is hydrolyzed during the process and the hydrolyzed ASA is detrimental to sizing and causes agglomeration in the process. ASA should be fed into the process by mixing it as efficiently as possible close to the headbox. When a good retention is aimed at, it is advantageous to dose the cationic starch in a position as close to the headbox as possible. Cationic starch is adsorbed unevenly to adsorbents of different type. It is adsorbed to adsorbents with a large specific area, such as fillers and fines, more strongly than to fibers. Yet, the influence of starch is different in fibers and in fines. In order to avoid uneven distribution of the starch, it should be added as dilute as possible in a position where the mixing is good.

Other substances used as retention chemicals, which have not been mentioned yet, are for example alum, PAC, polyethylenes and polyamines.

Thus, a common and often encountered problem with retention chemicals is the hydrolysis, where the chemicals in question react with water and loose their effect at a rate typical of each chemical. Thus, if the feeding of the retention chemical to the paper pulp could be optimized so that the chemical in question would be in contact with water for as short a time as possible, considerable savings could be made in chemical costs alone.

The retention takes place either as a mechanical or a chemical retention where the basic idea is to change the charge of the additional chemicals so that they would be adsorbed to the fiber as efficiently as possible. The charge changes while the process proceeds which may cause dissolving of the flocks which have already been formed and thus result in weakening of the efficiency of the chemical and thus overdosing. Thus, if the feeding of the retention chemical to the paper pulp could be optimized so that the chemical in question would be introduced in a location as close to the headbox as possible, considerable savings could be achieved in chemical costs, alone.

When discussing fillers in paper making contexts, fine mineral products are usually meant the size of which in most cases is 0.5-5.0 μm. The most important fillers are calcium carbonate and kaolin. Sometimes also titanium dioxide is classified a filler although its particle size is smaller (for example 200-300 nm) and the price very high compared for instance to calcium carbonate. Also talc is sometimes used as a filler. It is characteristic of most of the fillers that they are brought to the paper mill in powder or sludge form.

A filler which has become very popular, i.e. PCC (precipitated calcium carbonate) in turn is produced on site at the paper mill. PPC consists almost fully of the calcite crystal form of calcium carbonate. The starting material is often lime stone which in most cases is calcined to CaO. In the paper mill, water is added to the lime in order to produce lime milk Ca(OH)₂, after which carbon dioxide CO₂ is added as bubbles to the lime milk. The crystal form of the forming PCC particles can be controlled by using different temperatures in the manufacture. The PCC produced in the mill usually has a weak cationic colloidal charge whereas dried PCC has a negative (anionic) charge.

The purpose of the fillers is to fill the paper, in particular in situations where the paper must have high brightness. A certain type of PCC is used when a particularly high opacity and precise thickness of the paper is desired. The use of PCC as a filler is very much similar to that of the other calcium carbonate products. The fact that PCC is weakly cationic while the other minerals are anionic, must, however, be taken into account in view of the retention. Carefully planned retention systems, however, work with calcium carbonate fillers of both the types. In some cases where PCC and an adhesive, such as AKD (explained later), are used it is recommendable to add the PCC first to the paper pulp and after that the AKD. Then a colloidal material such as for example starch can coat the PCC particles whereby the AKD in turn adheres better to the starch.

Other fillers used are for example titanium dioxide, magnesium carbonate, calcium sulphate, barium sulphate, sodium silicate, aluminium trihydrate, magnesium hydroxide, or a combination of these.

Adhesives, examples of which are ASA (alkenylsuccinic anhydride=alcylene amber acid anhydride) and AKD (alkylketene dimer) are substances designed to prevent water from being absorbed to the paper. They are usually employed when producing paper in neutral or alkaline conditions. The main aims in using ASA are preventing the reactions (hydrolysis) taking place with water, even distribution and mixing of ASA into the paper pulp, and efficient retention to the product to be produced. The hydrolysis is prevented by preparing the ASA emulsion only as late as possible before the emulsion is mixed to the paper pulp. The pH of the cationic starch solution, which is used in preparing the emulsion, is decreased for example with alum. The purpose of the starch solution is to coat the ASA droplets so that they would not at once contact water. Prior art suggests adding the ASA emulsion at a position after the vortex cleaner in the short circulation, in other words the region preceding degassing and the headbox feed pump. Although the cationic starch coating around the ASA droplets to some extent contributes to the attaching of the adhesive to the fibers, an efficient retention system is still needed to retain the adhesive quickly in the web to be produced. Immediate retention is important as the adhesive is in any case bound to the fines and filler and if it does not retain in the web, it ends up in the water circulations and becomes hydrolysed. Hydrolyzed ASA in turn can cause flocks, running problems and deterioration of sizing.

Another known adhesive is AKD which is alkaline and manufactured synthetically of fatty acids. The most common form is a wax-like solid substance, which is dispersed in small particles in a solution containing a stabilizer. The stabilizer can be a cationic starch or any other cationic polyelectrolyte. AKD has a much less reactive character than ASA. When using AKD the paper produced is hydrophobic whereby typical end products are among other things various liquid containers and ink jet papers. The use of AKD is particularly recommended in situations where the paper should withstand moisture for long periods of time.

AKD is brought to the mill as a milky emulsion, whereby its use is fairly easy. As the reactivity of AKD is weaker compared to ASA, its use is also more flexible. Many paper manufacturers add AKD to high consistency pulp, in other words before dilution of the pulp to a consistency suitable for the headbox. In this way the AKD is brought to the surface of the fibers. On the other hand, if AKD is dosed to a pulp in a consistency suitable for the headbox it is justified to assume that it adheres mostly to the fines. If PCC is present it can decrease the efficiency of the adhesive and also with time reduce the effect of the adhesion for example during storage.

Paper manufacturers also speak about micro particles. These are for example colloidal silica, bentonite and some organic compounds which are used for the same purpose. All the micro particles commercially available at the moment have a negative colloidal charge and their specific area is very large. Micro particles are used to improve the dewatering properties of the fiber web. Usually they are added to the paper pulp after the cationic polyacrylic amide or cationic starch used as the retention chemical. In other words the polyacrylic amide or the starch is at first allowed to flocculate the fibers and the micro particles are added only after that to the paper pulp. The adding usually takes place to the headbox feed duct after the machine screen. It has been found that the best result is obtained when the whole system is made slightly cationic with the cationic additives before the micro particles are added. If the paper pulp is very anionic it should be treated with a cationic additive such as alum, polyaluminium chloride, polyamine or polyethyleneimine.

Further, depending on the case, very many different chemicals, antifoaming agents, optical brighteners, dyes and opacity pigments are used in paper machines, which aim at influencing the properties of the end products and/or improving the effect of other chemical and/or avoiding process problems. Examples of these are fixatives used to bind impurities in mechanical pulp. New pigments and their combinations influencing the paper brightness, saving of fibers, and paper structure, etc.

When looking into the problems that at the moment have been found in the paper manufacture and especially in the mixing of the chemicals and other additives in it, it is best to start with the retention chemicals as they have a central role in the whole additive program of the paper manufacture. The worst known problem associated with the retention chemicals has until now been the fact that it has not been possible to mix them in an adequately homogenous and quick way to the paper pulp. One has then been compelled to choose, quite naturally the alternative that secures the adequate dose of the retention chemical in the whole volume of the paper pulp flow running to the headbox by both overdosing the retention chemical and allowing it more time to be mixed to the paper pulp. In other words, the retention chemical is mixed with the pulp in most cases in the feed pump of the headbox, in the machine screen or immediately after the machine screen, in order to secure a flow time (=mixing time) long enough in the feed pipeline of the headbox. This has, however, had the consequence that on the other hand the retention chemical has lost some of its efficiency for example for the reasons associated with the evening out of the electric charges and chemical phenomena mentioned above and, on the other hand, due to the overdose, there have sometimes been complaints about the quality of the end product. It must be stated, however, that the long mixing time and the mixing distance provided for evening out the mixing which reduces the efficiency of the retention chemical has to some extent compensated the chemical overdose whereby the drawbacks have not been so imminent. Then there is, however, the danger that even a remarkable portion of the retention chemical is not retained in the web but becomes hydrolyzed and ends up with the filtrate of the wire section in the short circulation where it may for example cause precipitation. When talking about the minimum mixing distance of the mixing device, the distance is meant which the chemical or the corresponding substance needs to be mixed essentially homogenously to the pulp. With an efficient mixing device it is on the order of 1.5-2 seconds during which time, and also along the corresponding distance, the chemical is homogenously mixed to the pulp.

Further, it has been explained above how in connection with the feeding of an additive it has been found detrimental to feed the retention chemical to the paper pulp at a very early stage compared with the feeding of the additives. In many cases the tests we have performed have shown the best feeding method to be the feeding of the retention chemical and the additives at the same time to the paper pulp so that the retention chemical becomes at first mixed with the additive and essentially at the same time spreads to the paper pulp whereby in fact the entire mixing takes place in one second or a shorter time.

Chemicals used as paper additives are usually dosed in very small volumes. Feeding a small volume to a large volume homogenously is not successful if as efficient mixing as possible is not guaranteed at the feeding moment. If the mixing is poor, the chemical gets in contact with a small portion of the pulp suspension, only, and a remarkable portion of the pulp suspension remains without the chemical which is seen as variations in the properties of the end product.

Several different prior art methods and apparatus are known for feeding both retention chemicals and among other things the additives described above to the paper pulp. According to the conventional paper stock manufacturing method, both the various paper pulp fiber fractions and the additives, fillers, adhesives etc. required in the paper manufacture are brought to a mixing tank in the so-called short circulation. Also a part of the retention chemical/chemicals has/have conventionally been introduced to the mixing tank. In the mixing tank, as also the name suggests, the paper pulp is efficiently mixed so that both the different fibers and the various additives are mixed homogenously and the consistency of the suspension formed of these is adjusted to a desirable level. From the mixing tank the paper pulp is pumped by means of the headbox feed pump towards the head box in most cases via vortex cleaning, gas separation and a headbox screen or the so-called machine screen. Both the feed pump in question and the headbox screen mix the pulp further, in other words they keep the paper pulp as homogenous as possible. I most cases a retention chemical is fed to the paper pulp after the headbox screen with the intention to ensure the retention of a certain or some additive/s, filler/s or adhesive/s of the paper pulp in the paper machine wire section.

A very weak additive retention has been found to be a problem in the prior art short circulation process. In a test performed the additive retention (so-called first pass retention) was found to be in a conventional process arrangement on the order of five percent. In other words only five percent of the additive in the paper pulp remained in the web produced while the rest ended up in to the white water and the filtrates of the press section. However, these filtrates are recycled to the manufacture of paper pulp, whereby the additives which were not retained can end up in the paper machine but it is quite as well possible that they in several other connections end up in the reject. In a conventional process the additive is added to the mixing tank where also the white water and other usable filtrates are brought and from which the paper pulp is pumped via a vortex cleaning plant and a headbox screen to the headbox of the paper machine. In other words both the vortex cleaning plant and the headbox screen reject some of the paper pulp which always contains also some additive. Additives can also be different in reactivity and thus they can for example be hydrolyzed and precipitated at a point in the process which results both in an additive loss and problems in the process both because of fouling and detaching of the deposits which takes place from time to time.

When trying to mix both the retention chemicals and the additives homogenously to the paper pulp, an apparatus could be used, which has proved to be very efficient in particular in the mixing of bleaching chemicals of the pulp industry, in other words a mechanical revolving mixer of the type described in U.S. Pat. No. 5,279,709, which could be placed in the feed pipeline of the headbox, preferably between the machine screen and the headbox. There are, however, a few drawbacks in the use of this apparatus. Firstly, in order to ensure efficient and homogenous mixing, mechanical mixers in general develop a very strong field of shear forces, which breaks weak chemicals such as polymer chains of some retention chemicals. Of course this problem does not exist if the chemical to be mixed is not sensitive to shear forces. A mechanical mixer still has other weaknesses. These are for example the high price and the high operating costs because a mixer capable of mixing the chemicals homogenously over the whole diameter of the headbox feed pipe is large and it consumes a huge amount of energy while performing the mixing action. Further, the installation of the mechanical mixer to the pipelines and the drive motor on a stand of its own and constructing the electrical connections required involves a lot of work and supplies. For example installation to an existing paper machine requires that stands are built on the floor of the mill for the mixer and its drive. A further requirement is that the headbox feed duct is cut and a piece of it cut out so that flanges can be welded in the remaining ends of the duct if reductions or expansions are not needed for the flanges of the mixer, which would further encumber the work. The mixer can then be installed between these flanges.

An installation work almost as complicated is required by the use of a mixer based on the use of contoured members suggested for some applications where in the same way a piece is cut off from the flow duct leading to the headbox and a piece of a pipe containing contoured members is installed it its place, the purpose of the contoured members being to create turbulence in the paper pulp flowing to the headbox. The apparatus in question thus comprises a pipe replacing a part of the paper pulp feed duct, inside of which pipe there are arranged a number of contoured members, so-called turbulence elements. The retention chemical is fed in connection with the elements mentioned so that turbulence created in the flow by the contoured members is supposed to mix the chemical evenly to the paper pulp. It is disclosed that the apparatus is in particular used in the feeding of a two-component retention chemical to a paper pulp flow. The apparatus may be used also in feeding other chemical or additives to a paper pulp. In some cases the contoured member has several feed openings all of which can be used for feeding the same substance/chemical or some opening for feeding another substance/chemical.

The apparatus in question is mentioned to achieve a very gentle mixing which for example does not break the weak molecular chains of the polymer-type retention chemicals as badly as other prior art apparatus. The gentle nature of the chemical feed is among other things ensured by actually not spraying the chemical to the paper pulp flow but by just allowing them to flow to the paper pulp flow duct at a pressure only that much higher than the process pressure that the feed flow is in general possible. According to our understanding, however, the practise has shown that the turbulence created by the contoured members is in most cases too weak to mix the chemicals homogenously to the paper pulp flow. This is revealed among other things by the paper web being produced by the paper machine, in the quality of which there have been found fluctuations which cannot be explained otherwise. The reason can be for example that it is not possible to provide in a sensible way in the headbox feed duct a duct section containing the turbulence elements that would be long enough. Further problems may be the flow resistance caused by the turbulence elements, which changes the power requirement of the headbox feed pump and possibly the local pressure fluctuations caused by the elements, which can be reflected up to the headbox.

In view of the total economy, the best retention chemical mixing apparatus has been disclosed for example in U.S. Pat. No. 6,659,636 B1, the installation of which only requires drilling fairly small holes in the wall of the flow duct. Because the apparatus in question does not contain moving mixing elements there is no need for separate stands for the drive motor but the apparatus may be installed directly in support of the flow duct. As additional apparatus for controlling the flow, valves are naturally needed as in all chemical mixers irrespective of their type. The operation of the apparatus in question is based on spraying by means of a feed liquid the retention chemical to the paper pulp flow duct through one or several nozzles located at the periphery of the flow duct, whereby the high speed of the feed liquid causes the retention chemical to spread in a fan-like spray throughout the whole paper pulp volume flowing in the duct.

The purpose of the present invention is to solve for example the following problems

-   -   hydrolysis of different chemicals due to too early mixing,     -   change of the electrical properties of the retention chemicals         due to too early mixing,     -   high investment costs         -   a mixer of its own for each chemical         -   each mixer to a different location in the short circulation         -   large apparatus         -   powerful electric motors     -   high operating costs caused by the powerful electric motors,     -   high installation costs         -   constructing the stands         -   cutting the headbox feed duct         -   electrical installations required by the drive motor of the             mixer

Among other things in order to overcome the drawbacks described above the method and apparatus to be described below has been developed, the characteristic features of which are disclosed in the appended patent claims.

The tests we have performed have shown that the mixing of the chemicals and/or additives of the paper making industry is successful if the mixing measures are performed in the right order and so that, if necessary, an intermediate result of the mixing is a homogenous mixture of paper pulp and chemicals, or at least the end result is a homogenous mixture to be introduced to the headbox.

For example it has been noticed that by using an apparatus, the operating principle of which was described already above and in the U.S. Pat. No. 6,659,636 B1, in a new way for a slightly different purpose than before, an optimal situation can be reached in the mixing of chemicals and additives of the paper making industry where the volume of the chemicals to be used is remarkably reduced at the same time as also the quality of the end product is improved or at least remains easily at the desirable level.

Further it has been noticed in the tests performed that the feed liquid jet typical of the apparatus in question firstly mixes the chemical and/or additives fed with it homogenously to each other already at the spraying stage so that it is justified to speak about two almost simultaneous mixings. Firstly, the chemicals and/or additives supplied with the jet are mixed both with each other and with the solids and/or chemicals possibly carried by the feed liquid. And secondly, simultaneously with the mixing in question the feed liquid jet spreads evenly the material fed to the paper pulp flowing to the headbox. In order to secure this, several feeding apparatus are provided at the periphery of the paper pulp flow duct if necessary. Thus for example the retention chemical and the filler may be and advantageously is fed by means of the same feeding apparatus to the paper pulp. In a corresponding way, also the adhesive and the starch/polymer may be introduced via the same feeding apparatus and the retention chemical via another feeding apparatus a little later; in practice this distance in the paper pulp flow direction need not be more than about two meters.

The method and the apparatus of the invention provide among other things for example the following advantages:

-   -   efficient and homogenous mixing of additive to the paper pulp     -   quick mixing of additive and retention chemical to each other     -   an essentially improved additive retention     -   reduced investment, installation and operation costs, and     -   lower chemical costs

only to mention a few advantages.

Thus, the method and the apparatus of the invention are applicable in all processed where various chemicals must be introduced. As advantageous examples of the processes, among others fiber suspension processes of paper mills, thickening processes of various sludges, recycled fiber processes and bleaching processes may be mentioned, and in general processes where it is necessary to feed chemical to a filtrate, fiber suspension, sludge or a corresponding medium.

In the following, the method and the apparatus according to the invention are described in more detail with reference to the appended drawing figures, where

FIG. 1 illustrates the conventional prior art short circulation arrangement of a paper machine,

FIGS. 2 a, 2 b and 2 c illustrate three different variations of a conventional prior art feeding apparatus,

FIG. 3 illustrates a short circulation process arrangement according to a preferred embodiment of the invention, and

FIG. 4 illustrates a feeding apparatus according to a preferred embodiment of the invention.

According to FIG. 1, the prior art short circulation process arrangement works so that paper pulp to be fed to a paper machine, which is generally illustrated by a wire section 22, is diluted to the applicable consistency in a wire pit 20 with white water from the paper machine 22, although a separate mixing tank may also be utilized. Other suitable liquids may be used for dilution too, if desired, as for instance filtrate from a white water filter. Thus, both various fiber fractions 14, which the paper to be manufactured is desired to contain and various additives and filler 16, the use of which both saves valuable fibers and gives the paper desired properties such as for example brightness/opacity, gloss, moisture resistance, etc are brought to the wire pit 20. All these and possibly also at least a portion 18 of the retention chemicals are mixed in the wire pit with a mixer suitable for that purpose, to form a homogenous suspension. From the wire pit 20, the fiber suspension is taken by means of a pump 24, which further agitates the suspension, to a vortex cleaning plant 26 and further to a gas separation tank 28. The gas-free fiber suspension is pumped by means of a headbox feed pump 30, which also agitates the suspension, to a headbox screen 32, the so-called machine screen, which is used in addition to screening also for mixing the paper pulp, and after which the only retention chemical 38, or if a two-component retention chemical is used, the second component is added to the fiber suspension by means of a feeding apparatus 34 before the fiber suspension reaches a headbox 36 of the paper machine 22. Retention chemical has in known arrangements been fed also to various other positions in the short circulation between the wire pit and the head box.

FIG. 2 a illustrates an apparatus solution known per se described in the U.S. Pat. No. 6,659,636 already mentioned above. The feeding apparatus 34 according to the figure is, in fact, a nozzle comprising a casing 50 (illustrated here as being conical), flanges 52 and 54 disposed in it and preferably, but not necessarily, placed at its opposite ends, and a conduit 56 for the retention chemical. The feeding apparatus 34 is connected by its flange 52 to the feeding liquid duct and by its flange 54 to the flow duct taking paper pulp to the headbox of the paper machine. In the arrangement according to the figure, the casing 50 of the feeding apparatus 34 is converging, which by no means is absolutely necessary either in view of the structure or the operation of the device, from the flange 52 towards the flange 54 inside of which there is an opening 58 of the feeding apparatus. A purpose of the conical form of the casing 50, or other corresponding means including adjustment of the feed pressure of the liquid to be introduced, is to accelerate the medium flow in the feeding apparatus 34 so that the velocity of the jet discharging from the feeding apparatus 34 into the fiber suspension flow is at least three times, but preferably about five times the velocity of the fiber suspension flow. A velocity difference of this kind can ensure that the liquid jet discharged from the opening 58 penetrates quickly enough and deep enough into the fiber suspension flow, and is mixed with the fiber suspension essentially more homogeneously than with apparatus used before. In the embodiment according to FIG. 2 a, the retention chemical feeding conduit 56 to the feeding apparatus 34 is preferably tangential in order to ensure that the retention chemical discharging through the opening 58 of the feeding apparatus 34 into the fiber suspension flow is distributed homogeneously at least over the whole periphery of the opening 58. At the same time, tangential feeding ensures that the retention chemical is mixed into the feed liquid under as small shear forces as possible in order to prevent the polymer chains of the chemical from degrading.

FIG. 2 b illustrates another apparatus embodiment partly known from the U.S. Pat. No. 6,659,636 already mentioned above. Firstly, in the feeding apparatus 34 of the figure, the feed liquid inlet flange 52′ has been arranged, unlike in the solution of the patent mentioned, at the side and a feed connection 62 for the chemical, which may be for example a retention chemical, directly above the feed opening 58. Further, in this figure the chemical feed connection 62 is illustrated to extend as duct 64 inside the feeding apparatus 34 close to the feed opening 58, by means of which it is possible, if desired, to ensure that the retention chemical does not contact other substances before the mixing itself.

FIG. 2 c illustrates a feeding apparatus 34 according to FIG. 2 a, in fact with two additional embodiments. Firstly, inside the feeding apparatus 34 there is a centrally disposed hollow member 80 into which the retention chemical is supplied via the conduit 56. In this embodiment, the member 80 essentially comprises two rotationally symmetrical shells 82 and 84 and possibly one end wall 86 illustrated here as being conical. Further, at the end of the member 80 on the fiber suspension flow duct side, there is preferably an annular opening 88 provided, via which the retention chemical is allowed to be discharged into the fiber suspension. The retention chemical conduit 56 pierces the wall of the casing 50 of the feeding apparatus 34 and further leads via the annular space 90 between the casing 50 and the member 80 into the member 80 through the outer shell 84, at the same time preferably carrying the member 80 in its place. The inner shell 82 defining the member 80 is cylindrical and forms or includes a duct 92 which may be of two different structures. Contrary to what has been illustrated in the figure, the inner shell 82 may end at the level of the end wall 86 of the member 80 whereby, while the upper end of the inner shell 82 is open, some of the feed liquid flowing from the feed duct secured to the flange 52 may be discharged to the fiber suspension flow. In this embodiment, the retention chemical flow guided tangentially into the member 80 turns into a spiral flow towards an annular opening 88 of its own, via which the retention chemical is discharged as a fan-shaped jet into the fiber suspension together with the feed liquid discharging both from outside the opening 88 in this embodiment via the annular opening 58, and from inside the opening 88 via a duct 92. An additional purpose of the member 80 is to further throttle the cross-sectional flow area of the mixing apparatus in order to ensure a sufficient velocity difference between the retention chemical flow and the fiber suspension flow. A second purpose of the member 80 is to enable the mixing of the retention chemical with the feed liquid to take place essentially at the same time as the retention chemical is fed into the fiber suspension flow. The figure clearly shows that the retention chemical need not necessarily be in any contact with the dilution liquid before it is discharged through its opening 88 into the fiber suspension flow duct.

In another embodiment illustrated in FIG. 2c, the inner pipe 92 of the member 80 is connected to the process via a flow path 94 of its own and the outer pipe of the apparatus 34, forming the wall of the casing 50, via a flow path 96 of its own. Both flow paths 94 and 96 have been provided with flow regulation devices 98 and 100, preferably valves, as naturally has been done also with all the liquid connections of all the previous embodiments although this has not been illustrated in FIGS. 2 a and 2 b. The flow pipe 96 functions the way already presented before, but it is now possible to introduce into the inner pipe 92 of the member 80 e.g. either clean water, circulation water from the paper mill, white water, clear filtrate or some other non-clean liquid suitable for that purpose, even fiber suspension to be fed into the headbox of the paper machine. In other words the flow path in question is used in feeding so-called mixing liquid to the apparatus, the liquid being discharged to the chemical to be fed essentially at the same time as the chemical is discharge to the feed liquid and further to the pulp flow. It is of course possible, if it is desirable to use the mixing liquid mentioned to dilute the chemical, to arrange the inner pipe 92 to end at a distance from the opening 58 whereby the mixing liquid has some time to dilute the chemical.

Further, it is possible to introduce via the flow path 94 a retention chemical component, if desired, especially in case of a retention chemical containing several components. As an example, a short-chained retention chemical might be mentioned, in case the retention chemical is formed of a long-chained and a short-chained chemical. In that case, the long-chained chemical is supplied tangentially into the member 80 through the conduit 56 illustrated earlier in FIGS. 2 a and 2 b.

Fl patent application 2003051 illustrated a further prior art feeding apparatus which is to a large extent based on the basic structure of a feeding apparatus illustrated already in FIG. 2 c. Here, the flow paths for different liquids have been designed in a slightly different way and particularly the structure of the feed end of the inner duct, in this case the dilution liquid feed duct (corresponds to the duct 92 and its feed end in FIG. 2 c) is clearly different from the ones disclosed earlier. The feed end of the duct is actually closed but there are a number of nozzle openings provided at the sides of the duct through which the dilution liquid to be introduced from the duct can be discharged evenly all around to the feed liquid flowing at a high speed outside the duct. The basic idea in the use of the nozzle openings is to spray and mix the chemical coming from the feed duct (corresponds to member 80 in FIG. 2 c) outside the feed duct mentioned (corresponds to duct 92 in FIG. 2 c) efficiently to the feeding liquid coming from a duct surrounding the chemical feed duct, just before the feeding liquid with the substance added to it is in turn mixed efficiently and smoothly to the paper pulp or corresponding material flowing in the flow duct.

Fl patent application no. 20031468 further discloses a prior art feeding device which differs from the apparatus presented in the earlier embodiments in that at the end of the feeding apparatus flow duct, corresponding to the member 80 in FIG. 2 c, facing the paper pulp feeding duct there is disposed a space into which the chemical to be supplied in small volumes is introduced via a central duct. Now the chemical is thus introduced via the innermost duct and the dilution liquid via the duct located next in the direction towards the periphery. The space in question is defined by the duct bringing the dilution liquid from the outside so, that the duct in question is practically closed. The innermost duct bringing the chemical extends close to the closed end of the dilution liquid duct so that the chemical, while flowing from its duct under pressure against the end of the dilution liquid, spreads homogenously to the space where also the dilution liquid is introduced. In this way the chemical is distributed to the dilution liquid after which the mixture produced is discharged via the openings provided in the side surface of the dilution liquid duct to the feed liquid flowing outside preferably at a high speed. Naturally also in this embodiment all the feeding apparatus ducts mentioned above have been provided with regulating valves so that each flow can be adjusted independently irrespective of the other flows.

FIG. 3 illustrates a process arrangement which the invention tries to apply as efficiently as possible. FIG. 3 correspond otherwise to FIG. 1 but here only fiber fractions 14 are fed to the mixing tank/wire pit 20 and the additives of the paper manufacture are fed by means of apparatus 34 after the headbox screen 32. However, it should be noted that the apparatus 34 must be understood in a very broad way. It may also denote several apparatus disposed at a distance from each other both in the peripheral and in the longitudinal direction of the duct. The most essential thing, however, is that an essential portion of the additives 38-42, preferably all of them, are introduced to the paper pulp after the headbox screen. Then all the added substances quickly end up onto the paper machine wire whereby the solids fed do not have for example an opportunity to end up in the rejects either in the vortex cleaning or in the headbox screen. Thus, the result is a quick response to the adjustment whereby ash control in the paper is improved essentially.

In the following, apparatus are discussed with which the mixing of chemicals of the type in question is successful so that the result is a homogeneous mixing of the chemicals to the paper pulp and also so that the chemicals have not had time for example for harmful reactions either with each other or for instance with water.

In principle most of the apparatus applying the method of the invention are modifications of the feeding apparatus of the U.S. Pat. No. 6,659,636 already mentioned earlier and also illustrated in the above FIGS. 2 a, 2 b and 2 c. A simple feeding apparatus employing the method of the invention greatly resembles the feeding device presented in FIG. 2 a. However, it differs from it to some extent, because a preferred feeding apparatus employing the method of the invention has in addition to the retention chemical feed connection also another feed connection for the additive, chemical or a corresponding substance via which the additive mentioned is brought to the feeding device. In a process according to a preferred embodiment of the invention, so-called filler is introduced via the feeding connection mentioned, which may be titanium dioxide, talc, kaolin, calcined kaolin, calcium carbonate, PCC, magnesium carbonate, calcium sulphate, barium sulphate, sodium silicate, aluminium trihydrate, magnesium hydroxide or a combination of these. According to another preferred embodiment of the invention, ASA adhesive, AKD adhesive or corresponding substance is fed in via the connection mentioned. In addition to the fillers or adhesives the process of the invention can be used for feeding two or multicomponent chemicals to the paper pulp, such as retention chemicals which are for example composed of a micro particle component and a polymer component. Micro particles are for example colloidal silicon or bentonite. All the substances mentioned above are below generally called additives.

The feeding connection may be positioned at a suitable location in the casing of the feeding device, preferably tangentially in relation to the casing either in the same or in the opposite direction compared to the retention chemical feeding duct already earlier in the device. On the other hand, it may also be arranged in connection with the duct bringing feed liquid to the feeding apparatus close to the feeding apparatus. The maximum distance from the feeding apparatus of course depends on the chemical introduced via the connection in question and on the feeding liquid used. In other words if it is desirable for one reason or another not to allow the chemical and the liquid to contact each other before the actual mixing to the paper pulp the chemical must be introduced as late as possible to the feeding liquid. In practice it is, however, simplest to arrange the connection mentioned in the casing of the feeding apparatus whereby the feed liquid line leading to the feeding apparatus does not need any T-connections.

When using the feeding apparatus 34 of FIG. 2 a, the feeding liquid coming to the apparatus from above can be paper pulp taken via a branch pipe from the flow duct leading to the headbox, white water, or a filtrate suitable for this purpose or even clean water. In the feeding apparatus in question the retention chemical is fed to the feeding liquid via the first connection and the additive mentioned above via another connection so that in practise they can contact each other only inside the feeding apparatus just before the feeding liquid jet penetrates to the paper pulp flowing in the flow duct. The actual mixing does not take place until while spraying both the retention chemical and the additive mentioned with the feeding liquid to the flow duct.

The following short table illustrates the results of a test run performed in a paper mill. In the test a conventional method of feeding filler was compared with the feeding method according to the invention described above

test 1 test 2 test 3 test 4 TiO₂ feed g/ton 25 25 20 20 Opacity 91.7 91.5 92.0 92.3

Tests 1 and 2 relate to a conventional method where the filler is fed to the paper pulp already in the wire pit or in the headbox feed pump. Tests 3 and 4 on the other hand relate to a method according to the invention where the filler is introduced to the paper pulp substantially at the same time as the retention chemical and is mixed to the paper pulp by means of a strong feeding liquid jet close to the headbox after the machine screen.

The table indicates that when 20 g/ton of titanium dioxide is fed by the method of the invention, in other words 5 grams, i.e. 20 percent less than in the conventional method, the opacity readings were in fact higher than with the conventional process using more filler. It is also possible to calculate from the opacity readings obtained in the tests, which filler feed amount would, using the method of the invention, give the same average opacity values as the ones in tests 1 and 2. The calculation indicates that a dose of 15 g/ton is adequate, which in practise corresponds to a 40 percent smaller filler feed amount when using the method of the invention, compared with the conventional method.

Why then does the process of the invention save the additive, in this case titanium dioxide, i.e. the opacity pigment. The explanation is believed to be that when earlier the filler was dosed in the mixing tank or a corresponding member to the paper pulp and the retention chemical either at the same time to the same mixing tank or, as another alternative somewhere around the headbox screen to the pulp, the retention chemical came in both cases into contact mainly with the fibers whereby most of the retention chemical was consumed in binding the fibers to each other and a smaller portion of the chemical was left free for the filler. When the retention chemical and the filler are now fed in one turbulent jet to the paper pulp the retention chemical molecules and the filler particles have a better chance of meeting each other. Then a greater part of the filler particles can adhere to the retention chemical which in turn adheres to the fibers whereby the filler retention as a whole improves essentially.

Another example of the superior characteristics of the method and the apparatus of the invention compared to the prior art technology is a test where ASA adhesive was fed to paper pulp both in the conventional way and according to the invention. When the conventional method was used, about 5 percent of the ASA adhesive was retained at the paper machine wire section and when using the method of the invention, the percentage was about 35. The explanation to this phenomenon is probably the same as above.

When using the feeding device illustrated in FIG. 2 b the connection for feeding the second chemical may be located as in the feeding apparatus of FIG. 2 a. In other words, the feeding connection of the second chemical is located at the side of the feeding apparatus; via this connection the additives mentioned above or combinations of them can be fed to the feeding apparatus. Of course also in this modification, a second connection for the chemical, if the chemical allows it, can be provided already on the side of the feeding duct attached to the flange and bringing feeding liquid to the feeding apparatus.

When using the structural alternatives illustrated in FIG. 2 c as a starting point for using the method of the invention, the possibilities for introducing the additional chemical/chemicals increase. Then it is possible to arrange also in the outer wall of the feeding device casing a connection for the additional chemical which like in the previous embodiments can be an additive, an adhesive or any other chemical used in the paper manufacture which can be fed to the feed liquid just before the headbox. In a corresponding way, the chemical in question can be fed to the feed liquid already before the upper flange of the feeding apparatus.

Another alternative of introducing additional chemical is to arrange another feed connection for another chemical inside the inner member of the feeding apparatus whereby two different chemicals are brought at the same time inside the member. This can be done if the contact between the chemicals does not disturb their reactions. The way can very well be applied for example in feeding ASA adhesive and starch or ASA adhesive and polymer solution. When feeding these, is can even be thought that the starch or polymer solution is fed to the ASA immediately before the ASA arrives to the feeding device. Another possible way is to divide the inner member for example with an axial plane in two whereby two different chemicals can be fed irrespective of each other via the member.

A third possible way of feeding a second chemical is to introduce it via the central and innermost duct illustrated in FIG. 2 c either as a mere chemical or as mixed into a suitable feeding/dilution liquid or a corresponding medium.

A preferred method according to the invention is illustrated in FIG. 4 which applied the simple feeding device described in the US patent mentioned above. The feeding apparatus illustrated in FIG. 4 is composed in principle of a feeding apparatus 34 according to FIG. 2 b, why not also according to FIG. 2 a, and a feeding connection 68 disposed upstream of it in the wall of the flow duct 70 leading to the paper machine headbox. In other words the second chemical is fed via the feeding connection 68 to the flow duct 70 with such a small pressure difference that it hardly can assume its flow space against the wall of the flow duct 70. Then the chemical flows along the wall of the flow duct 70 where there is at a short distance from the feed connection 68 of the second chemical provided a feeding apparatus 34 via which the second chemical and the feeding liquid are sprayed to the paper pulp flowing in the flow duct. While the second chemical flows to the strong feeding liquid jet discharging from the feeding device it spreads efficiently and homognously to the paper pulp quite as if it had been fed from the feeding apparatus 34 itself the way illustrated in the previous figures.

Of course also a situation is thinkable, where several of the additional chemical feed connections 68 are disposed one after the other in the wall of the flow duct 70. In this case it should firstly be ensured that the chemicals introduced via these connections are such that their contact with each other even in high concentrations is not harmful to the chemicals themselves or to the paper pulp surrounding them at least at the center of the flow duct. Further, the feeding point in question must be so close to the feeding apparatus 34 developing the feeding jet that the jet of the feeding device is able to spread to the pulp flow all the chemicals fed from the upstream side. In practise this means that the mixing jet discharching from the feed apparatus 34 must be wider than the area to which the chemical flow/chemical flows discharging from the feed opening 68 have had time to spread when they reach the feeding apparatus 34.

On the other hand it should be noted that the apparatus of FIG. 4 can still be simplified so that it is used to feed one chemical, only, whereby only the so-called feeding liquid is brought to the feeding apparatus and the chemical or corresponding additive is introduced upstream of the feeding apparatus from which the chemical flow travelling with the paper pulp flow is fed to the paper pulp in the way described in connection with FIG. 4.

In the FIG. 4 above, only an example has been described of the kind of a feeding apparatus that could be used in connection with an additional chemical connection 68 provided upstream of it. However, one must immediately remember that all the feeding apparatus described above either in the figures or in text, only, are applicable in connection with the additional chemical connection. The only precondition of the use of the connection is the strong feeding liquid jet discharged from a feeding apparatus following it, by means of which the additional chemical coming from the connection is caused to penetrate to the desired depth into the paper pulp flow. Thus it is for example possible that a first chemical is fed from the connection 68 to the paper pulp and later a second or also a third chemical by means of the feeding apparatus.

Both the feeding apparatus illustrated by using figures illustrating prior art, and the feeding apparatus described in FIG. 4 can be used also in the feeding of, among other things, chemicals, such as for example retention chemicals, micro particles, fillers, binding agents, adhesive, optical brighteners, paper dyes, and silicates, to the flowing process liquid, only to mention a few chemicals. The feed device is thus applicable in all the processes where these chemicals must be fed, in particular when the chemical volume is small compared with the total volume of the flowing suspension flow. As advantageous examples of the processes, among others fiber suspension flows of paper mills, thickening processes of various sludges, recycling fiber processes and bleaching processes may be mentioned, and in general processes where it is necessary to feed chemical, particularly in very small amounts, to a filtrate, fiber suspension, sludge or a corresponding medium.

In addition to the chemical combinations mentioned above, titanium dioxide and some other suitable flocking chemical carried by the mixing liquid to the apparatus should be mentioned as an example of the first chemical to be fed. Another alternative is to feed silicate as the chemical and a filler, for example titanium dioxide, with the mixing liquid. Still a third alternative is to feed ASA adhesive as the chemical and bentonite in the mixing liquid.

In the mixing device according to the invention, the feed liquid by means of which a chemical is supplied to the process liquid, for example to a fiber suspension, can be the same fiber suspension, into which the chemical is to be fed. Of course also more dilute suspensions, various filtrates or corresponding media or mere fresh water are suitable for use as the feed liquid in the apparatus of the publication. Thus all the liquid obtained from another process stage that can be used in the feeding of the chemical at the same time saves fresh water and reduces for example the fresh water consumption of the mills.

As a summary, a situation will be described below in which the method of the invention utilizes the feeding device described in the U.S. Pat. No. 6,659,636 above. It is simplest to think the feeding apparatus in question to comprise three feeding ducts one inside the other. The so-called mixing liquid is conventionally fed to the paper pulp flow via the innermost of these, the chemical via the one in the middle and the feeding liquid via the outermost duct. Now according to the method of the invention, in addition to the conventional chemical, chemicals can be fed for example as follows:

-   -   1. the first additional chemical in the mixing liquid,     -   2. the first additional chemical in the feeding liquid,     -   3. the first additional chemical separately to the pulp flow         upstream of the feeding apparatus the way illustrated in FIG. 4,     -   4. the first additional chemical mixed into the feeding liquid         and the second into the mixing liquid,     -   5. the first additional chemical mixed into the feeding liquid         and the second separately to the pulp flow upstream of the         feeding apparatus the way illustrated in FIG. 4,     -   6. the first additional chemical mixed into the mixing liquid         and the second separately to the pulp flow upstream of the         feeding apparatus the way illustrated in FIG. 4,     -   7. the first additional chemical mixed into the feeding liquid,         the second into the mixing liquid, and the third in the         conventional chemical     -   8. the first additional chemical mixed into the feeding liquid,         the second to the mixing liquid and the third separately to the         pulp flow upstream of the feeding apparatus the way illustrated         in FIG. 4,     -   9. the first additional chemical mixed into the feeding liquid,         the second to the mixing liquid, the third in the conventional         chemical and the fourth separately to the pulp flow upstream of         the feeding apparatus the way illustrated in FIG. 4.

In the above description it must been noted that both the terms “ . . . in the liquid” and “mixed in the . . . liquid” must be given a broad interpretation. In other words the terms cover both mixing of the addition substance to the flow coming to the feeding apparatus and the feeding of the additional substance separately to the feeding apparatus and mixing it there into the liquid mentioned.

It has been noticed in the tests we have performed among other things the following chemical and/or additive combinations when fed with the same feeding apparatus essentially simultaneously to the headbox feed duct after the machine screen give a remarkably better result than previous mixing methods:

-   -   Retention chemical—filler     -   Retention chemical—micro particle     -   Retention chemical—adhesive     -   Silicate—filler     -   ASA adhesive—polymer

Further, it should be remembered that only examples have been presented above of the many chemical variations which can be introduced to a liquid by the method and the apparatus of the present invention. The starting point is, however, that the chemicals fed from one and the same mixing device should preferably be such that their contact with each other would not be harmful. This is particularly important when the chemicals are fed via the same flow duct of the mixing apparatus to the process liquid. This is easiest to avoid when working with chemicals reactive in relation to each other so that the different chemicals are taken to the process liquid via different flow ducts of the mixing apparatus. However, if the chemicals in question are highly reactive with each other, it is best to feed them from separate mixing apparatus disposed at an adequate “safety distance” from each other. 

1. A method of feeding chemical into a process liquid flow said method comprising: spraying a chemical to said process liquid flow with a feeding liquid jet created a feeding apparatus and having a transverse direction in relation to said process liquid flow whereby the chemical is mixed the feeding liquid substantially simultaneously with said spraying of the chemical, and feeding at least one of a further chemical and additive with the feeding liquid jet to the process liquid.
 2. A method as recited in claim 1, wherein said at least one of the further chemical and the additive is fed to said feeding apparatus before being introduced to the process liquid flow.
 3. A method as recited in claim 1, wherein said at least one of the further chemical and additive flows into a space in said feeding apparatus, wherein the space is separate from the mixing of feeding liquid and chemical.
 4. A method as in claim 1, wherein said at least one of the further chemical and additive flow into a space provided in said feeding device. wherein said space receives at least another chemical and additive.
 5. A method as in claim 1, wherein said at least one of the further chemical and additive flows into at least one of a feeding liquid space in said feeding apparatus and a flow duct feeding liquid to said feeding liquid space.
 6. A method as in claim 1, wherein said at least one of the further chemical and additive is added at a process pressure to a duct transporting said process liquid flow.
 7. A method as in claim 1, wherein said at least one of the further chemical and additive is added to a process liquid duct transporting the process liquid flow at a process pressure and is fed to the process liquid flow by feeding liquid jet discharged from the feeding apparatus.
 8. A method as in claim 1, wherein said process liquid includes a fiber suspension flowing in a paper machine headbox feed duct downstream of a machine screen.
 9. The method as recited in claim 8, wherein at least one of a fillers, binding agents, sizing agents, optical brighteners, antifoaming agents, two-component and multi-component chemicals is fed into the fiber suspension flow after the machine screen.
 10. The method as in claim 8, wherein the at least one of a retention chemical and filler, retention chemical and micro particle, retention chemical and sizing agent, silicate and filler, and sizing agent and polymer is fed into the fiber suspension flow after the machine screen.
 11. The method as recited in claim 9, characterized in that wherein said filler is at least one of titanium dioxide, talc, kaolin, calcined kaolin, calcium carbonate, PCC, magnesium carbonate, calcium sulphate, barium sulphate, sodium silicate, aluminium trihydrate, and magnesium hydroxide.
 12. The method as recited in claim 9, wherein said sizing agent is at least one of an ASA sizing agent, AKD sizing agent, polymer and corresponding substance.
 13. The method as recited in claim 9 wherein said retention chemical includes micro particles and polymers.
 14. The method as recited in claim 9 wherein said micro particle is at least one of colloidal silica, micro polymer and bentonite.
 15. The method as recited in claim 10 wherein said retention chemical is at least one of a cationic acryl amide copolymers, an anionic acryl amid copolymers, a colloidal silica, a micro polymer, bentonite, cationic starch, alum, PAC, a polyethylene, and a polyamine.
 16. A method as recited in claim 1, wherein a feeding point of said at least one of the further chemical and additive occurs such that a mixture of said at least one further chemical and additive and the paper pulp is substantially homogenous when the mixture arrives from the headbox to the wire.
 17. A method as recited in claim 1, wherein said at least one of the further chemical and additive is a retention chemical, a micro particle, filler, binder, sizing agent, optical brightener, paper dye and silicate.
 18. An apparatus for feeding chemical into a process liquid flow, the apparatus comprising: a feeding apparatus for feeding at least one chemical to said process liquid flow, wherein the feeding apparatus includes a nozzle spraying the chemical to the process liquid flow; the nozzle including a feeding liquid jet having a transverse direction in relation to the process liquid flow whereby the chemical is mixed with the feeding liquid substantially simultaneously with the spraying action, and a connection to the feeding apparatus for feeding additional chemical to a process liquid flow duct.
 19. An apparatus as recited in claim 18, wherein the process liquid is paper pulp and said connection is located in the flow duct of the paper pulp upstream of the feeding apparatus so that the chemical flowing from said connection to the flow duct is conveyed with the paper pulp flow to an opening of the feeding apparatus.
 20. An apparatus as in claim 18, wherein the feeding apparatus includes a feeding liquid space, a mixing liquid space and a chemical space into which each of the media mentioned is introduced by of flow ducts.
 21. An apparatus as in claim 18 wherein said connection is in flow communication with a feed liquid space of said feeding apparatus.
 22. An apparatus as in claim 18, wherein said connection is in flow communication with a mixing liquid space of the said feeding apparatus.
 23. An apparatus as in claim 18, wherein said connection is in flow communication with a chemical space said feeding apparatus.
 24. An apparatus as in claim 18, wherein said connection is in flow communication with a flow duct bringing medium to the said feeding apparatus.
 25. An apparatus as in claim 18, wherein said feeding apparatus and said connection for feeding additional chemical are flow communication with a paper machine headbox feed duct downstream of a machine screen. 